Unilateral conductive hearing loss (CHL) during development can produce long-term deficits in auditory functions including sound localization. The anatomical and physiological mechanisms responsible for and the time period over which these deficits occur are not well understood. A sensitive period for the development of binaural hearing is hypothesized to accommodate changes in the acoustical cues to location due to head and pinnae growth. While plasticity may allow calibration of neural circuits to the cues, it may also adapt to inappropriate cues resulting from CHL. The goals are to understand the concurrent development of the cues to location and the circuits that encode them in animals with normal hearing and those reared from birth with CHL. The primary focus of this proposal is to study the encoding of the interaural level difference (ILD) cue to sound location in adult animals with normal hearing and those that have had a mild to moderate unilateral CHL present from birth. In Specific Aim 1, extracellular recordings (both single- and multi-unit) will be used to study the monaural and binaural response properties of neurons in the central nucleus of the inferior colliculus (ICC), a midbrain nucleus in the ascending auditory pathway that receives afferent inputs from the brainstem nuclei that initially encode the cues to sound location. This aim tests the specific hypothesis that the acoustic coding capabilities of ILD-sensitive ICC neurons change in order to compensate for the altered ILD cues that result as a consequence of the unilateral CHL. To test this hypothesis, ILD sensitivity will be measured for pure-tone and noise stimuli and virtual space receptive fields will be measured for monaural and binaural stimulation under both normal hearing cue conditions and a "virtual earplug" condition that simulates those due to CHL. Specific Aim 2 explores how CHL due to fluid buildup in the middle ear (effusion) affects the entire ensemble of acoustic cues to location, the interaural time and level differences and the monaural spectral shape cues.